Interpretive Summary: Projections of world food supplies in a future atmosphere with elevated CO2 have been based primarily on studies that may not adequately account for co-occurring environmental stresses. Generally unbeknownst to the experimenter these stresses may reduce yields in what should be the control treatments of the experiment. Addition of CO2 to these plants may then moderate or eliminate the effects of the unrecognized stress and result in a substantial increase in yield which would then erroneously be attributed to a direct effect of CO2 on productivity rather than the real interaction with the unrecognized stress. The unrecognized stress may be in the form of atmospheric pollutants, drought, nutrient deficiencies or even extremes of normal weather. And the stress itself could be mild enough not to produce visible damage but still cause subtle physiological changes that result in yield losses. Failure to account for these interactions may lead to unrealistically large estimates of the CO2-fertilization effect. It is important when projecting world food supplies to account for these interactions.

Technical Abstract:
Studies indicate that increasing atmospheric CO2 levels enhance the productivity of current cropping systems. However, 8 years of experiments in open-top field chambers on seed crops (cotton, soybean, rice and wheat) exposed to reciprocal treatments of O3 and elevated CO2 suggest that some of these gains may be smaller than previously thought. Larger apparent gains depend on the co-occurrence of yield-lowering stresses, such as O3, and amelioration of their effects by elevated CO2. Yield increases from elevated CO2 in our experiments ranged from -26% to 40% in charcoal-filtered air while plants grown under O3 stress showed yield increases from doubled [CO2] of up to 140%. Even though photosynthetic enhancement was sustained in elevated CO2 resulting in increased biomass, it was not always translated into yield. Such a lack of a CO2 effect on seed yield while biomass increased is also reported for native species. In the case of O3, elevated CO2 ameliorated the average 27% yield suppression by reducing leaf conductance and limiting O3 flux into leaves. Absent recognition of the extant O3 stress, the yield increase would appear to be a direct stimulation by CO2. Similar unrecognized environmental interactions may arise between CO2 and water, nutrient or other stresses which may exaggerate the yield response to elevated CO2. These effects should be included in projections of productivity in a future elevated CO2 atmosphere.